SSC 440 Exam 1

Soil Science GIS

A GIS connects "where" something is with "what" it is.
What actually holds those two halves together?

A unique identifier that links each feature to one row in the attribute table

A single feature class can contain more than one kind of geometry. For example, one feature
class can hold both point features and polygon features.

False

You are making a map showing where soil samples were taken in a field. The field also contains a farm road, a drainage ditch and a power line. You leave all three off the map.

Which of the choices made during cartographic abstraction is that?

Selection

Corn acreage and swine operations are concentrated in many of the same North Carolina counties.
What has the GIS actually shown you?

That the two things are found in the same places

A raster attribute table holds one row per _______.

Unique cell value

A land cover raster has a cell size of 30 metres.
In its attribute table, the Forest class has a Count of 4,500.
How many acres of forest does that represent? 
(Use 1 acre = 4,046.86 square metres, and round to the nearest whole acre.)

1001

A digital color photograph stored as a raster holds three values at every cell location.
What do those three values represent?

The intensity of reflected light in three different parts of the electromagnetic spectrum

Applying a different stretch, or changing the colour ramp, changes the values stored in the
cells of a raster.

False

Multiplying the number of rows by the number of columns in a raster gives the total number of
cells in that raster, including any cells that hold NoData.

true

A layer contains 12 field polygons. You run Select by Attributes and 4 records are highlighted.

Which statement is true?

The stored data are unchanged, and clearing the selection leaves all 12 features in the layer.

A numeric field named Yield has nothing recorded at all for some records.

Which expression finds those records?

Yield IS NULL

Using the attribute table shown, how many records does this expression select?
(Crop = 'Corn' AND Acres > 15) OR Tillage = 'No-Till'

5

You have 6 fields selected from an earlier query. You now want to answer a completely unrelated question about the same layer.

What should you do first, and why?

Clear the selection, because a selection method can add to or subtract from the existing set rather than replacing it.

In Select by Location you set the soil samples as the input features and the fields as the selecting features, with the relationship 'within'.

Which layer ends up with highlighted records, and what happens if you swap the two?

The samples are highlighted. Swapping input and selecting asks a different question and highlights the fields instead.

A destination record has no matching record in the source table.

After the join, what happens to that record?

It stays in the output, with nulls in the fields brought over from the source.

The figure below shows a destination table and a source table, with the matches drawn between them. Reading from the destination records to the source records, what is the cardinality of this relationship?

one-to-many

The same one-to-many match can produce either one summarised row for each target, or one row for each matched pair.

Which statement is correct?

The join operation changes how many rows the output has. It does not change which features match.

Two fields come back from a summarised spatial join with the same mean pH. One has a Join_Count of 12 and the other has a Join_Count of 2.

What follows?

The two means are not equally well supported, so Join_Count matters as much as the mean does.

A layer contains 12 field polygons. You run Select by Attributes, 4 records are highlighted, and you then clear the selection.

How many features are in the layer now?

12, because a selection changes the current selection state and never alters the source data.

Which of these questions requires Select by Location rather than Select by Attributes?

Which soil samples fall inside Field E?

When you run Select by Attributes, what happens to each record in the table?

The expression is evaluated once for that record, and the record joins the selection if the result is TRUE.

A field named Crop stores text values such as Corn and Soybeans. A field named Acres stores numbers.

Which expression is written correctly?

Crop equals 'Corn' AND Acres is greater than 10, with quotes around Corn and none around 10.

Using the field table from Monday's lecture, one expression joined by AND returned a single record, while a different expression joined by OR returned three.

What general rule does this illustrate?

AND narrows a selection, because a record must satisfy every condition, while OR broadens it, because a record needs to satisfy only one.

You have 6 fields selected from an earlier query. You now want to answer a completely different question about the same layer.

What should you do first, and why?

Clear the current selection, because a selection method can add to, remove from, or select within the existing set rather than replacing it.

In Select by Location you set the soil samples as the input features and the fields as the selecting features, with the relationship "within".

Which layer ends up with highlighted records?

The soil samples, because the input layer is the one that receives the selection.

Why is "select the samples within the fields" a different operation from "select the fields containing samples"?

Because the relationship is read from the perspective of the input layer, so swapping input and selecting layers changes both the question and which layer is highlighted.

You have 5 fields selected and you want a permanent dataset containing only those 5.

What do you need to do?

Export the selected features, because selecting does not alter the source and exporting creates a separate dataset.

In a join, which dataset determines the rows and the geometry of the result?

the destination, also called the target, because it retains its rows and geometry while the source supplies related attributes.

Two tables are joined on a shared key. Which statement is true?

The key field names may differ between the two tables, but the key values must refer to the same entity.

A destination record has no matching record in the source table. What happens to it after an attribute join?

It stays in the output, and the fields brought in from the source are null for that record.

You run an attribute join, the tool reports success, and every new column is empty.

Where should you look first?

At the two key fields, checking for incompatible data types, inconsistent formatting or stray spaces, and missing values.

Cardinality is read from the destination records to the source records. Which pair of questions establishes it?

How many source records can match one destination record, and can one source record match several destination records?

Four destination records each match one source record, and two of them match the same source record.

What does the output look like?

Four rows, with the source attributes repeating on the two records that matched the same source.

Field E is the destination and it matches four soil sample records. Which statement describes the situation correctly?

It is not an error. You choose either one summarized row for Field E or one row per matched pair, and Join_Count records how many matched.

Why does the lecture say to name the target layer before anything else in a spatial join?

Because the target controls the geometry and the base records of the output, so it determines what the result means before the relationship is even chosen.

You are joining well locations to field boundaries. How do a nearest match and a within-a-distance match differ?

Nearest returns one match per target and can record the match distance, while within a distance returns every feature inside the threshold, so a target may get zero, one, or many.

Two fields come back from a summarized spatial join with the same mean pH. One has a Join_Count of 12 and the other has a Join_Count of 2.

What should you conclude?

The means are not equally well supported, and Join_Count, nulls, and duplicates all need inspecting before the result is used.

Based on the vector operation categories introduced in lecture, consider this task: you need the field boundary with a 300 ft stream setback removed. What must change in the output, and which category does that point to?

Geometry changes, input attributes are retained, and the category is extraction.

You clip a soils layer to a field boundary. The field boundary layer has an an attribute for crop type (i.e. Corn, Soybeans, etc.). What does the output contain?

The soil polygons cut to the boundary, carrying the soils layer's attributes only. Crop does not appear.

A perennial stream runs just outside a field. You are asked to identify the part of the field lying outside a 300 ft setback from that stream. You clip the streams to the field first, then buffer. What is wrong?

The nearby stream is deleted before it can contribute its setback, so the output keeps land that should have been removed.

The figure below shows an overlay operation, each with its inputs and its output. Name the operation.

intersect

After a Dissolve your output has 4 rows, but the map shows 19 separate polygons.

What does that tell you?

Some rows are multipart. One row stores several separated parts, and selecting that row selects every part.

Your fields layer has a user-created Acres field that was calculated before any processing. You erase three setback masks from the fields. The tool reports success and the map looks right, but Acres still reads what it did before the Erase.

What follows?

Acres was calculated on the old geometry and has to be recalculated, because a user-created area field does not update itself.

Which task requires a local operation?

Convert every elevation cell from meters to feet using the same conversion factor.

The conditional expression below was applied as a local operation to the raster shown.

Con(Layer1 < 2, 0, Layer1)

What value is written at the cell marked x?

3

A soils raster has 100 ft cells and a yield raster has 10 ft cells. You resample the soils raster to 10 ft so the two can be combined.

What is true of the result?

The output has 10 ft cells, but resampling does not add new information about soil variation.

Two students reclassify the same soil test P raster into three suitability classes. One uses published agronomic thresholds and the other uses quantile breaks.

What follows?

The maps may differ, because the two methods can assign different class boundaries.

A DEM contains one cell holding 90, and its eight surrounding cells all hold 10. You want that spike removed without pulling the neighboring cells toward it. Which 3 x 3 focal statistic should you use, and why?

Median, because the middle value of the nine cells is 10.

You want the mean yield within each of 14 management zones. In Zonal Statistics, which layer supplies the zones and which supplies the values?

The management zones are the zone layer and yield is the value raster, because the zones define the groups and yield supplies the measurements

A raster of land cover classes and a raster of ground elevation differ in what their cell values represent. Which statement describes them correctly?

Land cover values are discrete categories and elevation values are continuous measurements.

Which scope of raster analysis summarizes cells that share an identity, whether or not those cells are next to each other?

zonal

A raster of yield is multiplied by 1.2 in a local operation. What is true of the output raster?

It has the same cell size and the same cell locations, and only the values differ.

Two rasters named A and B hold measurements from two years. Someone writes the expression A + B / 2 intending to average them. What does that expression actually calculate at each cell?

The value from A plus half the value from B.

Which map algebra workflow produces a raster of ones and zeros that a later step can use to make a decision?

A comparison that tests each cell against a threshold.

A soil property was mapped from measurements collected at 100 foot resolution. The raster is resampled to 10 foot cells. What has the resampling produced?

Smaller cells and the appearance of more precision, with no additional real detail.

One raster is subtracted from another. The two cover overlapping but not identical areas, and each has some NoData cells inside its own coverage. Where does the output raster contain usable values?

Only at locations where both inputs hold valid values.

The same soil test phosphorus raster is reclassified into low, medium and high twice. The first time uses published agronomic thresholds. The second uses quantiles, which place an equal number of cells in each class. What should you expect?

Two different maps, because the class boundaries fall in different places.

You want a focal neighborhood to reach farther from the focal cell than it currently does. Which neighborhood setting do you change?

size

A moving window calculates a statistic from the cells inside it. Where is each result written?

At the focal cell's matching location in the output raster.

In a raster, one cell holds 90 and the eight cells around it all hold 10. A 3 by 3 focal mean is run. What happens to the value 90?

It is replaced by 18.9 at its own location, and it influences the nine output cells whose neighborhoods contain it.

A raster contains scattered single cells that are far higher or far lower than everything around them. You want those cells gone without blending the surrounding values together. Which focal statistic should you use?

median

Focal statistics, block statistics and resampling are each run on the same 20 foot digital elevation model. Which statement is correct?

Focal and block calculate new statistics and keep the cell size, while resampling moves values onto a grid with a different cell size.

Two moving window operations are run on the same image with the same window size. One softens boundaries and suppresses isolated extreme cells. The other amplifies local differences and makes boundaries stand out. What accounts for the difference?

The weights in the kernel

A digital elevation model stores elevation in feet, and its cells are 10 meters across. Slope is calculated without correcting for this. What happens?

A digital elevation model stores elevation in feet, and its cells are 10 meters across. Slope is calculated without correcting for this. What happens?

You want the mean yield within each soil map unit on a farm. Which layer supplies the groups and which supplies the measurements?

The soil map units are the zones, and the yield raster supplies the values.

You run Zonal Statistics as Table on a yield raster using field boundaries as the zones, and the tool finishes. What do you have, and what has to happen before you can shade the fields by mean yield?

A standalone table with one row per field, which has to be joined to the field features on the zone identifier.

Two watersheds both have a mean rainfall of 10 inches, but watershed B receives three times as much total water as watershed A. What explains that?

Watershed B covers three times the ground area, so the same average depth falls across far more cells.

Euclidean Distance and Euclidean Allocation are run on the same raster from the same two source cells. What does one cell contain in each output?

A distance to the nearest source in one output, and the identifier of the nearest source in the other.

A set of soil sample points is converted into a raster. What is actually given up?

The original feature records and their exact geometry, replaced by a grid of cells.

Two county layers both show valid North Carolina geometry, but they do not overlay. A point in Dataset A reads -78.67, 35.79 and a point in Dataset B reads 2,100,000, 740,000. Which explanation fits best?

They use different coordinate frameworks, and coordinates alone do not name their framework.

Williams Hall is described as 35.7867, -78.6715 in one spatial reference and 35.7865, -78.6717 in another that uses a different datum. Which statement is correct?

Both can be correct, because each describes the same place relative to a different datum.

An exaggerated image of the geoid shows highs and lows across the globe. What do those highs and lows represent?

Variations in a gravity-defined surface, caused by unevenly distributed mass inside Earth.

Why are coordinates calculated on an ellipsoid rather than directly on the geoid?

The ellipsoid is smooth and defined by two semi-axes, while the geoid is irregular.

An ellipsoid has been chosen for a coordinate system. What does a datum add that the ellipsoid alone does not provide?

The position and orientation of that ellipsoid relative to Earth.

A colleague says WGS 84 must be more accurate than NAD 27 for every task in North America, because WGS 84 is global. How should you respond?

Not necessarily, since a regional datum was fitted to its region and suitability depends on the task.

Two projects each carry the same one-meter datum shift. One is staking a buried tile drain line for excavation, the other is a statewide map of county crop totals. Where does the shift matter?

Only for the tile line, because only that task needs positions tighter than one meter.

A field record lists an elevation of 100 m but names no vertical datum. What is missing?

The surface that counts as zero, so the height has no defined reference.

A GPS receiver reports an ellipsoid height of 50.0 m at a point where the geoid height is -33.0 m. What is the elevation (orthometric height) at that point?

83.0 m

In the coordinate pair -78.67, 35.79 from a geographic coordinate system, which value is longitude and what does it measure?

-78.67 is longitude, an angle west of the prime meridian.

A point is recorded as 78 degrees 40 minutes 30 seconds W. Which is the correct signed decimal degree value?

-78.675

Is one degree of longitude longer on the ground at Raleigh or at the equator?

At the equator, because meridians converge toward the poles.

Besides its underlying geographic coordinate system, what does a projected coordinate system contain?

A projection method, its parameters, and linear units.

Which projection surface is the usual choice for a mid-latitude region that extends mostly east to west?

conic

The Mercator projection preserves local shape. What happens to area on a Mercator map as you move toward the poles?

Scale and area increase, so high-latitude regions appear larger than they are.

You are comparing the acreage of several watersheds. Which projection property matters most?

equal-area

Why might the coordinate 0,0 of a projected coordinate system for North Carolina sit outside the state?

False easting and northing offset the values, often to keep them positive.

Why might UTM be inconvenient for statewide North Carolina data?

The state spans three UTM zones, each with its own coordinate grid.

North Carolina's single State Plane zone uses Lambert Conformal Conic. Which statement explains the choice?

Its east-west extent suits a conic, and conformal preserves local shape and angles.

One project maps parcel boundaries in a single county and another compares crop acreage across the United States. How should their projected coordinate systems differ?

Parcels suit a local conformal system, and national acreage suits an equal-area one.